1. Field of the Invention
The field of the invention is that of digital transmission with frequency spreading, and in particular, although not exclusively, CDMA (Code Division Multiple Access) transmission. The CDMA technique, which consists in multiplying a source signal (included in a common frequency band) by means of a specific code, constitutes one application of frequency spreading.
2. Description of the Prior Art
In transmission systems of the above kind, frequency spreading modulation devices are generally used for transmission. They apply to input signals (source signals) frequency spreading followed by quadrature modulation to obtain the signals to be transmitted. Conventionally (and this applies in the remainder of the present description), the input signal or each input signal is deemed to have a bit rate D while the signal to be transmitted and the received signal each have a bit rate N*D, where N is the spreading factor.
The invention is more precisely concerned with coherent demodulation devices of the type for regenerating, from the signals received, the input signals of the aforementioned frequency spreading modulation devices.
In the context of the present invention, only complex frequency spreading (corresponding to the use of two spreading sequences in quadrature) is of interest. Real spreading (corresponding to the use of a single spreading sequence) has an inherent performance handicap such that it is of no interest.
Two families of frequency spreading transmission are generally recognized:
single-channel transmission: the modulation device receives a single input signal to which it applies frequency spreading followed by quadrature modulation to generate the signal to be transmitted; and PA1 multi-channel transmission: the modulation device receives a plurality of input signals and multiplies each of them by a separate orthogonal code (a Walsh code, for example) to obtain a plurality of channels. It combines this plurality of channels onto a single multi-channel signal to which it applies frequency spreading and then quadrature modulation to obtain the signal to be transmitted. PA1 quadrature demodulation means generating a demodulated signal at bit rate N*D from said received signal; PA1 complex despreading means generating a despread signal at bit rate N*D from said demodulated signal; PA1 means for summing over N samples generating a summed signal at bit rate D from said despread signal; PA1 a loop for estimating and correcting the phase shift induced in said demodulated signal, said loop comprising: PA1 means for regenerating said input signal from said summed signal, themselves comprising means for resolving a residual static phase ambiguity induced by application of said predetermined function. PA1 quadrature demodulation means generating a demodulated signal at bit rate N*D from said received signal; and PA1 complex despreading means generating a despread signal at bit rate N*D from said demodulated signal; PA1 a plurality of processing branches each associated with a given channel from said plurality and including: PA1 a loop for estimating and correcting the phase shift induced in said demodulated signal, said loop comprising: PA1 in each of said processing branches, means for regenerating said input signal of said given channel from said summed signal specific to said given channel, themselves comprising means for resolving a residual static phase ambiguity induced by application of said predetermined function. PA1 means for multiplying said estimate of the phase shift or said average estimate of the phase shift by a predetermined scalar quantity to adjust the dynamic characteristics of said loop; and PA1 means for integrating said estimate of the phase shift or said average estimate of the phase shift over a predetermined time period to obtain a cumulative estimate of said phase shift. PA1 the real input signals at bit rate D (x=1); and PA1 the complex input signals at bit rate D (x=2), each generated by a 1 to 2 serial/parallel converter from a real source signal at bit rate 2*D. PA1 means for sampling the real part if said input signal is a real signal; PA1 threshold means; and PA1 Viterbi decoding means if convolutional encoding means are used by the transmitter. PA1 phase shifting means for shifting the phase of the signal at the input of said Viterbi coding means by a value chosen from a predetermined set of values; and PA1 means for analyzing the signal at the output of said Viterbi decoding means, indicating to said phase shifter means the choice of one of the phase shift values according to the result of said analysis. PA1 0 and .pi. if said input signal or each of said input signals is a real signal; or PA1 0, .pi./2, .pi. or 3.pi./2 if said input signal or each of said input signals is a complex signal. PA1 means for combining said summed signals to obtain a final combined signal having a maximum gain; and PA1 means for resolving the residual static phase ambiguity of said final combined signal, induced by the application, on each of said diversity paths, of said predetermined function. PA1 phase shifter means for shifting the phase of a first of said summed signals or of a combined signal at the output of a preceding group of means by a value chosen from a predetermined set of values to generate a phase shifted summed signal; PA1 means for adding said phase shifted summed signal to another of said summed signals to generate a combined signal; and PA1 means for controlling said phase shifter means assuring a final choice of the phase shift value such that the combined signal has a maximal gain, a group of means receiving the combined signal generated by the preceding group of means only when said control means of said preceding group of means have effected said final choice,
In each of these two families, it is possible to distinguish two sub-families, respectively corresponding to the situations in which the input signal, or each input signal, is real or complex. A complex input signal at bit rate D generally results from passing a real signal at bit rate 2.D through a 1 to 2 serial/parallel converter.
The invention has many applications, for example in digital cellular mobile radio systems.
In cellular systems, single-channel transmission is typically used only for an uplink (mobile station to base station) channel where the mobile station is supposedly satisfied with the existence of a single communication channel to the base station. A plurality of mobile stations can each transmit in "single-channel mode" in the same frequency band. Because they use different spreading sequences or different phases of a common spreading sequence the base station can separate out the signals transmitted by the various mobile stations.
Moreover, in cellular systems, multi-channel transmission is typically used in the case of a downlink (base station to mobile station) channel where the base station has to communicate with a plurality of mobile stations. The signal transmitted by the base station is then an aggregate of several channels broadcast to all the mobile stations. These channels are separated by the use of codes known as "orthogonal" codes in that they enable a mobile station receiver to extract the channel addressed to it without this being impeded by the presence of other channels.
However, the current trend in standardizing future CDMA type cellular networks is to introduce multi-channel transmission in the uplink (mobile station to base station) direction as well. This trend is justified by the resulting flexibility (in particular for multimedia applications) and by the possibility of adopting coherent demodulation (offering higher performance than the non-coherent demodulation that has to be used in multi-channel receiving devices).
The existing demodulation techniques (with their respective drawbacks) are described below for each of the two transmission families previously mentioned.
First, it should be remembered that a demodulation (or receiving) device has the task or regenerating the input (source) signal or signals from the signal that it receives. The signal received corresponds to the signal transmitted affected by various disturbances. The type of disturbance of interest here is a phase shift. After the received signal is demodulated (using two carriers in quadrature), the resulting demodulated signal is a complex signal subject to a phase rotation. This rotation corresponds precisely to the phase shift. The phase shift is known to be due to the propagation medium and to the modulation and demodulation operations (and in particular due to the asynchronism of the local oscillators feeding the modulator and the demodulator). The phase shift varies in time, i.e. it is a dynamic phenomenon. The treatment of the phase shift varies for single-channel and multi-channel transmission.
In the case of single-channel transmission, non-coherent demodulation is currently used, which has repercussions for the receiver as well as for the transmitter. The principle of non-coherent demodulation is to choose a transmitted sequence that can be interpreted at the receiver without knowing the phase shift due to the channel.
Unfortunately, adopting non-coherent demodulation leads to a performance handicap.
In the case of multi-channel transmission, coherent demodulation is currently used, which presupposes a knowledge of the varying phase shift introduced by transmission, modulation and demodulation. The current solution to the problem of acquiring this knowledge consists in dedicating one channel to the transmission of a pilot signal. In other words an all "one" signal is generally transmitted as one of the input signals. The receiver exploits the presence of the pilot signal to estimate the channel and in particular to determine the phase shift due to the channel. Once it knows this, the receiver can cancel the phase shift.
Unfortunately, using a pilot signal also reduces system performance. The channel carrying the pilot signal is not available for transmitting data. Also, the pilot signal channel often has to have a higher power rating than a normal channel, in particular if the dynamic variations are fast. This surplus transmitted power does not convey any information and so link performance is degraded.
An objective of the invention is to alleviate the various drawbacks of the prior art.
To be more precise, one objective of the present invention is to provide a single-channel coherent demodulation device usable in the case of single-channel transmission and offering better performance than the conventional non-coherent demodulation devices referred to above.
Another objective of the invention is to provide a multi-channel coherent demodulation device usable in the case of multi-channel transmission and offering better performance than the conventional coherent demodulation devices using a pilot signal referred to above.
Another objective of the invention is to provide single-channel and multi-channel coherent demodulation devices of the above kind adapted to estimate and to correct the phase shift induced by the propagation medium in particular without using any hypothesis as to the transmitted signals and in particular without transmission of any pilot signal.
A complementary objective of the invention is to provide a receiving system using a plurality of diversity paths which retains the advantages associated with the (single-channel or multi-channel) devices included in the system.